(a) Field of the Invention
The present invention relates to an acrylic impact modifier having a core-shell structure, and in particular, it relates to an acrylic impact modifier prepared by a polymerization process of at least three stages, a process for the preparation thereof, and a poly(vinyl chloride) (PVC) containing it that has excellent impact resistance.
(b) Description of the Related Art
Methyl methacrylate butadiene styrenic (MBS) resins, chlorinated polyethylenic (CPE) resins, acrylic resins, etc. are used as impact modifiers to improve the impact resistance of poly(vinyl chloride). Of them, the acrylic resins have good weatherability and are thus widely used as impact modifiers for outdoor plastic products that are exposed to sunlight for a long time. As an example, for products such as window frames that require impact resistance and weatherability at the same time, polymers having a core-shell structure wherein methacrylic polymers having good compatibility with poly(vinyl chloride) are grafted onto a rubbery core consisting of alkyl acrylates, are mostly used.
As factors determining the physical properties of acrylic impact modifiers having a core-shell structure, the rubber content of impact modifiers, the size of rubber particles, the distance between rubber particles, the swelling index for solvent, the bonding degree between matrix and impact modifier particles dispersed by milling, and so forth, are important. In particular, the bonding between the matrix and the impact modifier is determined by the grafting efficiency of the shell onto the impact modifier rubber core.
As a process for preparing impact modifiers to improve the impact resistance of poly(vinyl chloride), there is conventional emulsion polymerization, which includes the following two methods. The first method, as shown in U.S. Pat. No. 5,612,413, is multi-stage emulsion polymerization carried out firstly by polymerizing a seed, then by charging core component monomers thereto over two to four stages to grow a rubber particle, and finally by wrapping the core surface by the addition of shell component monomers to complete a core-shell structure. The second method is, as shown in European Patent Publication 0 527 605 A1, a microagglomeration method wherein a latex having a core-shell structure of 100 nm or less is polymerized, it is allowed to grow to a desired particle size by agglomeration, and a final core-shell structure is formed by the formation of an encapsulation shell.
The multi-stage emulsion polymerization process, as shown in said U.S. Pat. No. 5,612,413, can be divided into core polymerization and shell polymerization. The core polymerization uses alkyl acrylate monomers. The alkyl acrylates exhibit the impact resistance of a rubber component due to their low glass transition temperature and the fact that double bonds in molecules do not remain after polymerization, and accordingly, they can prevent the decomposition of polymers from UV and the decrease of impact strength derived therefrom. In addition to alkyl acrylates, compounds having at least two non-conjugated carbon-carbon double bonds are used as a cross-linker. The cross-linkers of rubber not only maintain the stability of latex, but they also help the structure of the impact modifier rubber to retain its shape within the matrix during the milling process. If cross-linking of the rubber is insufficient, the stability of the latex cannot be secured and uniform dispersion does not occur, thereby causing a decrease of impact resistance and weatherability. Also, if the degree of cross-linking of rubber is too high, impact strength decreases.
Shell polymerization is conventionally conducted by graft polymerizing onto the surface of a core of the methyl methacrylate monomers, which have excellent compatibility with poly(vinyl chloride), alone or together with a small amount of monomers having two or more functional groups. In particular, the methyl methacrylates not only have excellent compatibility with the matrix, but they also have a comparatively high glass transition temperature, and accordingly they contribute to the improvement of the coagulation properties of the latex. To increase the dispersability of impact modifiers in the interior of the matrix, acrylonitrile monomers may be added in a small amount during the shell polymerization.